Re: Is shmem page accounting wrong on split?

[Yang Shi <shy828301@gmail.com>]

On Fri, Aug 28, 2020 at 7:55 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Fri, Aug 28, 2020 at 03:25:46PM +0100, Matthew Wilcox wrote:
> > If I understand truncate of a shmem THP correctly ...
> >
> > Let's suppose the file has a single 2MB page at index 0, and is being
> > truncated down to 7 bytes in size.
> >
> > shmem_setattr()
> >   i_size_write(7);
> >   shmem_truncate_range(7, -1);
> >     shmem_undo_range(7, -1)
> >       start = 1;
> >       page = &head[1];
> >       shmem_punch_compound();
> >         split_huge_page()
> >           end = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE); # == 1
> >           __split_huge_page(..., 1, ...);
> >             __delete_from_page_cache(&head[1], ...);
> >       truncate_inode_page(page);
> >         delete_from_page_cache(page)
> >           __delete_from_page_cache(&head[1])
> >
> > I think the solution is to call truncate_inode_page() from within
> > shmem_punch_compound() if we don't call split_huge_page().  I came across
> > this while reusing all this infrastructure for the XFS THP patchset,
> > so I'm not in a great position to test this patch.
>
> Oh, this works for truncate, but not hole-punch.  __split_huge_page()
> won't call __delete_from_page_cache() for pages below the end of the
> file.  So maybe this instead?
>
> It's a bit cheesy ... maybe split_huge_page() could return 1 to indicate
> that it actually disposed of the page passed in?

I'm fine to have split_huge_page() return 1.

>
> +++ b/mm/shmem.c
> @@ -827,7 +827,7 @@ static bool shmem_punch_compound(struct page *page, pgoff_t start, pgoff_t end)
>                 return true;
>
>         /* Try to split huge page, so we can truly punch the hole or truncate */
> -       return split_huge_page(page) >= 0;
> +       return split_huge_page(page) >= 0 && end < -1;

It would be more clear if we could have some comment about what "-1"
means. It took me a little while to understand the magic number, but
once I understood it it looks more straightforward to me.

>  }
>
>  /*
>
>